1. Field of the Invention
The present invention relates to piezoelectric thin film devices using a piezoelectric thin film, more specifically to piezoelectric thin film devices including, on an Si (silicon) substrate, a piezoelectric thin film of an alkali niobium oxide-based perovskite material.
2. Description of Related Art
Piezoelectric materials are used for piezoelectric devices of various applications. For example, they are widely used for functional electronic components such as actuators in which an applied voltage deforms a piezoelectric element thereby providing an actuation function, and sensors for detecting a physical quantity by utilizing, conversely to actuators, a voltage generated by a deformation of a piezoelectric element. As piezoelectric materials for use in such actuators and sensors, there have been widely used lead-based dielectric materials with excellent piezoelectric properties, in particular perovskite structure ferroelectric materials expressed by the general chemical formula: Pb(Zr1-xTix)O3 (often called PZTs). A PZT is typically made by sintering an oxide of its constituent metals.
In the trend toward downsizing and increasing performance of electronic components, there is also a strong demand for piezoelectric devices with smaller size and higher performance. However, as a piezoelectric material made by widely used conventional sintering methods becomes thinner, the following problem comes to the fore. Specifically, as the thickness of a piezoelectric material approaches the order of 10 μm, it becomes comparable to the grain size of the piezoelectric material; therefore, the influence of the grain boundaries can no longer be ignored. This produces problems such as fluctuation in piezoelectric properties and accelerated device degradation. In order to solve such problems by replacing conventional sintering methods, fabrication methods of piezoelectric materials such as those utilizing thin film formation techniques have been researched in recent years. Therefrom, there have been reported PZT films sputtered on an Si substrate for use in high-sensitivity gyro sensors (angular velocity sensors) (e.g., see JP-A-2005-203725).
On the other hand, PZT piezoelectric sintered bulks and PZT piezoelectric thin films contain approximately 60 to 70 mass % of lead; so, it is desired to promote research and development of lead-free piezoelectric materials from an environmental consideration. Various lead-free piezoelectric materials are currently being studied, among which is potassium sodium niobate expressed by the general chemical formula: (K1-xNax)NbO3 (0<x<1) (hereinafter also referred to as KNN). A KNN has a perovskite structure and exhibits relatively excellent piezoelectric properties among lead-free piezoelectric materials, and is therefore expected to be a promising lead-free piezoelectric material candidate. A KNN piezoelectric material has excellent piezoelectric properties near x=0.5. And, there is a report that a KNN thin film epitaxially formed on an MgO single crystalline substrate (instead of an Si substrate) exhibits good piezoelectric properties (see Nonpatent Document 1).
Nonpatent Document 1: T. Mino, S. Kuwajima, T. Suzuki, I. Kanno, H. Kotera, and K. Wasa: Jpn. J. Appl. Phys., 46 (2007) 6960.
Such KNN thin films have been attempted to be formed on an Si substrate by other film formation methods such as sputtering and PLD (pulsed laser deposition). However, up to now, KNN thin films on an Si substrate exhibit a relatively low piezoelectric constant d31 compared to PZT thin films, and therefore have yet to be applied to high-sensitivity sensors such as gyro sensors. Moreover, in the above Nonpatent Document 1, the KNN piezoelectric thin film is formed on an MgO substrate; however, use of MgO substrates presents a cost disadvantage since they are expensive compared to silicon substrates.